1. Field of the Invention
The present invention relates to a communication system and a communication apparatus that are configured to be connected to a general electric receptacle and for power line communication and, more particularly, to a communication system and a communication apparatus that are configured for such mobile devices operating not on commercial AC (Alternative Current) power supplies as PDAs (Personal Digital Assistants) and mobile phones to execute power line communication.
2. Description of the Related Art
Recently, as means for building a LAN (Local Area Network) in a simple manner in buildings such as homes, PLC (Power Line Communication) has been put into practice that a device having communication capabilities and supplied with an electric power through a power line multiplexes a communication signal with a the power line to communicate with other devices having the similar capabilities through the power line.
Although the wireless LAN has been widely spreading as means for building LANs in a simplified manner, the wireless LAN presents problems of the difficulties in room-to-room communication caused by the poor wall-barrier surpassing characteristic due to the restricted transmission output power to avoid interference with various radio-associated regulations and other systems. In contrast, with the power line communication, use of existing power lines allows communication between devices installed in different rooms having AC receptacles, thereby makes ignorable the locations of mate devices. The power line communication allows to build a LAN without arranging Ethernet (registered trademark) in a building and can realize the high-speed communication of 100 Mbps or higher.
Referring to FIG. 12, there is shown a home network system based on a related-art PLC. In the figure, reference numeral 100 denotes a household power line, reference numerals 101 and 106 denote household receptacles, reference numerals 102 and 107 denote AC plugs, reference numerals 103 and 108 denote PLC modems (the PLC modem 103 being the master), reference numeral 109 denotes a host device such as a personal computer, reference numeral 104 denotes an optical line terminator device, and reference numeral 105 denotes the Internet. Reference numeral 104 may denote an ADSL (Asymmetric Digital Subscriber Line) modem.
The PLC modem 103 and the PLC modem 108 communicate with each other via the power line 100, allowing the PC (Personal Computer) 109 to be connected to the Internet 105 via the optical line terminator device 104.
A communication system based on the power line communication can realize a so-called ubiquitous environment that allows the use of the capabilities of computers having similar performance wherever users move. For example, a system is proposed in which power line communication capabilities are given to each kind of household devices such as household appliances, the household devices are monitored and controlled by a master device based on a PC for example, and the master device is accessed through an external public communication network for externally monitoring and controlling the household devices (refer to Japanese Patent Laid-Open No. 2005-143026 for example).
Because the transmission route of PLC is a power line, the network configuration is of bus type in the case of existing wiring, in which all communication devices in home that are connected to the power line are shared in band on a time division basis. In the system configuration shown in FIG. 12, there are two PLC modems; thus it practicable to arrange a plurality of modems for mutual communication.
As seen from FIG. 12, a related-art power line communication system is basically used by devices, such as desktop PCs, that are each mainly powered by a commercial AC power supply and have an AC plug that can be inserted in an AC receptacle. In other words, if mobile devices, such as PDAs (Personal Digital Assistants) and mobile phones, are in a battery driven state in which a commercial AC power supply is not used, these mobile devices cannot execute power line communication. In this case, it is required for a mobile device having wireless LAN capabilities to be connected to PLC via an access point or for a mobile device to be connected to an AC receptacle via a PLC modem or an AC adaptor.